1. Field of the Invention
The present invention relates to squarylium-metal chelate compounds which are suitable for recording materials in optical recording media, and to optical recording media comprising the squarylium-metal chelate compounds.
2. Description of the Related Art
Recordable DVD media have been developed as optical discs with a large capacity. The possibilities of increasing the recording capacity of the DVD media are in the development of a recording material that can minimize a recording pit to be formed, introduction of image compression technologies such as Moving Picture Experts Group-2 (MPEG 2), and improvement in the method of shortening the wavelength of the semiconductor laser used to read the recording pit.
An AlGaInP laser diode with a wavelength of 670 nm is only one red semiconductor laser that has been developed and commercialized for a bar code reader and a measuring instrument. Along with the development of the optical discs with a high recording density, the red semiconductor laser has been introduced and used in practice in the optical recording industry. For a driving system for a digital versatile disc (DVD), laser diodes with wavelengths from 645 nm to 675 nm are standardized as the light sources. A reproduction-only DVD-ROM drive equipped with a light source with a wavelength of about 650 nm is now commercially available.
Under such circumstances, archival recordable DVD media must have excellent light resistance and storage stability and be capable of recording and reproducing information by an optical pickup system using a laser beam having a wavelength of 645 nm to 675 nm.
To achieve the above performance, recording materials that form recording pits upon irradiation with light must be developed. Optical recording media using a variety of organic dyes have been developed. A squarylium compound can be given as an example of the materials used for a recording layer of an optical recording medium which carries out recording and reproducing information by the light pickup using a laser having a wavelength of 645 nm to 675 nm. The conventional squarylium compound, however, is significantly deteriorated by light irradiation, is not stable and cannot record information stably at the wavelengths ranging from 645 nm to 675 nm.
Highly reflective DVD media using a dye-containing recording layer (DVD+R and DVD−R media) utilize multiple reflection at the interface of a dye film. For yielding both satisfactory recording sensitivity and reflection ratio, the birefringence of the dye film must be controlled within proper ranges, i.e. a refractive index n of the recording layer alone of 1.5 to 3.0 and an extinction coefficient k of 0.02 to 0.3 with respect to light having a wavelength in a range of the recording-reproducing wavelength ±5 nm.
To satisfy the optical properties of the recording layer, the absorption edge in longer wavelengths of optical absorption in the dye film may be used as a recording-reproducing wavelength in DVD+R, DVD−R, and CD-R media, as shown in FIG. 1. However, according to this technique, the optical properties at the above wavelengths of laser often vary due to a large dependency on wavelength at the absorption edge.
As a possible solution to increase the light resistance of a recording layer, a formazan-metal chelate compound is exemplified as a highly light-resistant dye. However, a recording layer containing this dye alone generally has a low absorption coefficient and does not yield the aforementioned suitable birefringence.
A variety of organic dyes are used for the recording layer, and examples of optical recording media using these organic dyes are as follows:
(1) Write-once, read-many-times (WORM) optical discs such as (i) those using a cyanine dye (Japanese Patent Application Laid-Open (JP-A) No. 60-89842 and No. 61-25886); and those using a phthalocyanine dye (JP-A No. 63-37991 and No. 63-39888).
(2) Overwritable compact discs (CD-Rs) such as (i) those using a cyanine dye in combination with a metal reflective layer (JP-A No. 02-13656 and No. 02-168446); (ii) those using a phthalocyanine dye in combination with a metal reflective layer (JP-A No. 05-139044); and (iii) those using an azo-metal chelate dye in combination with a metal reflective layer (JP-A No. 08-231866 and No. 08-295811).
(3) Overwritable digital versatile discs (DVD-Rs) with a large capacity such as (i) those using a cyanine dye in combination with a metal reflective layer (JP-A No. 10-235999, and Pioneer R&D vol. 6, No. 2, 1996); (ii) those using an azomethine dye in combination with a metal reflective layer (JP-A No. 08-283263 and No. 10-273484); (iii) those using an azo-metal chelate compound and a metal reflective layer (JP-A No. 10-36693, No. 11-12483, and No. 2001-322356); (iv) those using a styryl dye in combination with a metal reflective layer (JP-A No. 11-144313 and No. 11-165466); (v) those using a formazan dye in combination with a metal reflective layer (JP-A No. 10-337958 and No. 2001-23235); and (vi) those using another dye in combination with a metal reflective layer (JP-A No. 10-309871 and No. 10-309872).